[iOS] Define a runtime feature flag for LinearMediaPlayer

https://bugs.webkit.org/show_bug.cgi?id=269210
rdar://122814889

Reviewed by Eric Carlson.

Defined a LinearMediaPlayerEnabled web preference with a default value of false. When the
preference is enabled, deactivated the code path in HTMLMediaElement that requires element
fullscreen for video fullscreen.

* Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml:
* Source/WTF/wtf/PlatformEnable.h:
* Source/WebCore/html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::enterFullscreen):

Canonical link: https://commits.webkit.org/274486@main

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -3962,6 +3962,21 @@ LegacyOverflowScrollingTouchEnabled:
     WebCore:
       default: true
 
+LinearMediaPlayerEnabled:
+  type: bool
+  status: unstable
+  category: media
+  humanReadableName: "Linear Media Player"
+  humanReadableDescription: "Enable LinearMediaPlayer for fullscreen video"
+  condition: ENABLE(LINEAR_MEDIA_PLAYER)
+  defaultValue:
+    WebKitLegacy:
+      default: false
+    WebKit:
+      default: false
+    WebCore:
+      default: false
+
 LinkModulePreloadEnabled:
   type: bool
   status: stable

Source/WTF/wtf/PlatformEnable.h

@@ -981,3 +981,8 @@
     && USE(EXTENSIONKIT)
 #define ENABLE_EXTENSION_CAPABILITIES 1
 #endif
+
+#if !defined(ENABLE_LINEAR_MEDIA_PLAYER) \
+    && USE(LINEARMEDIAKIT)
+#define ENABLE_LINEAR_MEDIA_PLAYER 1
+#endif

Source/WebCore/html/HTMLMediaElement.cpp

@@ -6863,6 +6863,9 @@ void HTMLMediaElement::enterFullscreen(VideoFullscreenMode mode)
 #if ENABLE(FULLSCREEN_API) && ENABLE(VIDEO_USES_ELEMENT_FULLSCREEN)
 #if PLATFORM(IOS_FAMILY)
     bool videoUsesElementFullscreen = document().settings().videoFullscreenRequiresElementFullscreen();
+#if ENABLE(LINEAR_MEDIA_PLAYER)
+    videoUsesElementFullscreen &= !document().settings().linearMediaPlayerEnabled();
+#endif
 #else
     constexpr bool videoUsesElementFullscreen = true;
 #endif